Final answer:
An atom can have the same number of energy levels and mass but a different number of electrons if it is an isotope of the same element, such as carbon-12, carbon-13, and carbon-14.
Step-by-step explanation:
An atom can have the exact same number of energy levels and mass, but a different number of electrons if it is an isotope of the same element. Isotopes are atoms of the same element that have the same number of protons (which determines the atomic number) but a different number of neutrons.
One example is carbon, which exists as three isotopes: carbon-12, carbon-13, and carbon-14. All three isotopes have the same number of energy levels and mass, but a different number of electrons due to the different numbers of neutrons.